Switch to yarn for installing dependencies

This seems more intuitive to me today, and I hope will make it easier
for contributors who want to add or upgrade plugins.

Note: this is using nightly yarn, because there's a fix for Circle CI +
Docker + Yarn compatibility that is made but not officially released
yet. We can switch to stable yarn once 0.18.0 leaves pre-release. The
fix was PR 1837 on the Yarn repo.

CONTRIBUTING.md

@@ -1,6 +1,26 @@
 # Contributing
 
+## Adding and upgrading plugins
+
+When adding or upgrading a plugin, we should see changes in both `package.json`
+and `yarn.lock`.
+
+Make sure to run `make image` before running these commands.
+
+Add a plugin:
+
+```
+bin/yarn add eslint-config-google
+```
+
+Upgrade a plugin:
+
+```
+bin/yarn upgrade eslint-config-google
+```
+
 ## Testing Changes
+
 Changes made to the engine can be tested locally.
 
 1. Install [the CodeClimate CLI](https://github.com/codeclimate/codeclimate).

Dockerfile

@@ -2,11 +2,13 @@ FROM node:6.5.0-slim
 MAINTAINER Code Climate <[email protected]>
 
 WORKDIR /usr/src/app
-COPY package.json npm-shrinkwrap.json /usr/src/app/
+COPY package.json yarn.lock /usr/src/app/
 
-RUN apt-get update && \
-    apt-get install -y git jq && \
-    npm install && \
+RUN apt-key adv --fetch-keys http://dl.yarnpkg.com/debian/pubkey.gpg && \
+    echo "deb http://nightly.yarnpkg.com/debian/ nightly main" | tee /etc/apt/sources.list.d/yarn-nightly.list && \
+    apt-get update && \
+    apt-get install -y git jq yarn && \
+    yarn install && \
     git clone https://github.com/eslint/eslint.git && \
     ESLINT_DOCS_VERSION=`npm -j ls eslint | jq -r .dependencies.eslint.version` && \
     cd eslint && \

Makefile

@@ -1,4 +1,4 @@
-.PHONY: image test citest shrinkwrap
+.PHONY: image test citest
 
 IMAGE_NAME ?= codeclimate/codeclimate-eslint
 
@@ -10,7 +10,3 @@ test: image
 
 citest:
 	docker run --rm $(IMAGE_NAME) sh -c "cd /usr/src/app && npm run test"
-
-shrinkwrap: image
-	docker run --rm --workdir /usr/src/app $(IMAGE_NAME) sh -c \
-	  'npm shrinkwrap >/dev/null && cat npm-shrinkwrap.json' > npm-shrinkwrap.json

bin/yarn

@@ -0,0 +1,6 @@
+#!/bin/bash
+
+set -e
+
+IMAGE_NAME=${IMAGE_NAME:-codeclimate/codeclimate-eslint}
+docker run --rm --volume "$PWD:/usr/src/app" "$IMAGE_NAME" sh -c "cd /usr/src/app && yarn $*"